1. Field of the Invention
The present invention relates to an anti-lock control method for preventing wheels of a vehicle from locking during braking, and more particularly, to an anti-lock control method for preventing the wheels from yawing during braking in the situation where either the right wheel or the left wheel runs on a high road surface and the other wheel runs on a low road surface.
2. Discussion of the Prior Art
Generally, for the purpose of maintaining good steering capability and vehicle stability while braking a vehicle, and also of reducing the braking distance, an anti-lock control device is used. Conventionally, the device includes a control unit having a microcomputer, where the control mode of the brake hydraulic pressure is determined according to electrical signals representing wheel speeds detected by wheel speed sensors. Hold valves (normally-opened solenoid valves) and decay valves (normally-closed solenoid valves) are opened and closed to increase, decrease or hold the brake hydraulic pressure.
In such a conventional anti-lock control method, a pressure decrease reference speed Vr is set based on a wheel speed to be controlled (hereinafter referred to as "system speed Vs") in each brake control system. A speed, which has a predetermined acceleration/deceleration following limit with respect to the highest one of the four wheel speeds, is set as an estimated vehicle speed Vv. There is also set a threshold speed VT which, while kept lower in speed a predetermined value than the estimated vehicle speed Vv, follows this estimated vehicle speed. In accordance with the comparison of the system speed Vs with the reference speed Vr or the threshold speed VT to determine which of them is higher, and also in accordance with detected high and low peak points of the system speed Vs, the time of starting each control mode (i.e. increasing, decreasing or holding the brake pressure), as well as the time of ending each control mode, are set. A plurality of hydraulic control valves (solenoid valves) are turned on and off in accordance with predetermined control procedures set respectively for these control modes, thereby increasing, holding and decreasing the pressure.
For example, a pair of control systems are set in such a manner that the wheel speeds of the right and left front wheels are represented by system speeds Vs1 and Vs2, respectively, and their anti-lock controls are independent of each other. In such an anti-lock control method, there is a condition when the pressure increases with respect to the right and left control systems, using the same mode judgment standard when the vehicle is running on a split road surface (i.e. a right wheel is on a high road surface and a left wheel is on a low road surface, or vice versa), in which the road surface friction coefficients .mu. (hereinafter referred to as "road surface .mu.") to which the right and left wheels are subjected are different from each other. Thus, in the control system in which the front wheel is on the high road surface, the brake hydraulic pressure is controlled such that a sufficient braking force can be obtained utilizing this high .mu. road surface. Whereas in the control system in which the other front wheel is on the low road surface, the brake hydraulic pressure is controlled such that the braking force is decreased to prevent a wheel from locking. The amount of pressure increase required for one cycle of acceleration and deceleration of the wheel is much larger than the low road-side control system. Therefore, a yawing moment develops in the vehicle body, which affects the vehicles stability.